People keep pets such as cats and dogs for a number of reasons. In urban areas many people keep their pets indoors for all or part of the day, particularly at night and prevent them going outside. This may be to prevent the animal from wandering and becoming lost, to prevent the animal killing native animals or for other reasons.
This leads to the animal being unable to go outside to urinate or defecate. The usual solution to this is to provide a tray with absorbent material into which the animal may urinate or defecate. This has a number of disadvantages, including odour, the need to regularly remove faeces and the need to periodically replace the absorbent material.